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When to repair a hairline crack in a top or back?
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Author:  Corky Long [ Sun Feb 24, 2019 6:24 pm ]
Post subject:  When to repair a hairline crack in a top or back?
The steel strings that I built were made 7 or 8 years ago. And now some of them are cracking, either in the tops on in the backs. Some of this is due to neglect on the part of their owners, rearding lack of humidity of their environment, and some is undoubtedly a combination of that and building them a bit light, or inconsistently. These are cracks along the grain, of spruce, Koa, or Mahogany, and they don't seem to affect the sound of the instrument. Braces remain in place, no issues with glue integrity or structural integrity. Just wood that got a bit too dry, and opened a seam.

When do you make the decision to repair these? Part of me is thinking that they will just do the same thing next year. (Live in New York - humidity swings between winter and summer are a fact of life.) Yeah, I know about the products one can put in the guitar case, etc. I also know my friends, who play my guitars - they love them, play them, but are likely to get caught up with life rather than obsess.

This is not an issue of warrantees or other customer service matters. I don't sell my guitars. These were gifts. This is more a question of what you think about the longevity of the instrument and the necessity (or lack thereof) of closing those gaps.

Thanks in advance.
Author:  meddlingfool [ Sun Feb 24, 2019 6:56 pm ]
Post subject:  Re: When to repair a hairline crack in a top or back?
Sooner the better IMO, and there's no real reason not to. Even if you don't take it as far as refinishing, it's at least good to close the cracks...
Author:  Clay S. [ Sun Feb 24, 2019 8:41 pm ]
Post subject:  Re: When to repair a hairline crack in a top or back?
If you don't repair them the wood will shrink and swell as the humidity changes. As it does this the cracks will become wider because the wood will shrink more than it swells. If you use titebond or hide glue you will not need to refinish the guitar if you don't mind minor cosmetic flaws. The cracks should not only be glued but cleated as well. This will help keep them from reopening.
If there are big gaps you may need to splint in a piece of wood (plane shaving). This will also help keep cracks from reappearing.
Most of the old war horses have had some work done to them over the years. I have seen some with holes knocked in them that still sounded great. If your friends are still playing the guitars then they are worth patching up.
Author:  Hesh [ Mon Feb 25, 2019 6:18 am ]
Post subject:  Re: When to repair a hairline crack in a top or back?
Very small cracks may not impact anything and can be fine left alone at times. Other cracks can travel and get worse or create an asymmetry to the support system for something else such as a bridge and help that lift too.

Then there is the issue of a crack relieving stresses in a top or back and if the instrument suffered from dimensional instability from either how it was made or what it was made with fixing one crack can result in more stresses and new cracks somewhere else, often the other side but not always.

So with all this said the idea that stresses in the structure should be addressed too some cracks are better off filled instead of forced closed creating stresses somewhere else in the structure.

For us if our efforts to rehumidify an instrument and then let it equalize back at 45% for a couple days does not result in a closed crack we likely will fill the sucker so as to not induce more stresses somewhere else.

But again as mentioned some cracks don't matter depending on where they are, what the wood is, etc.

I just happy that I got though this post with out any really bad jokes about cracks...... :). Whoops, nix that...
Author:  dpetrzelka [ Mon Feb 25, 2019 10:17 am ]
Post subject:  Re: When to repair a hairline crack in a top or back?
The other thing to consider, aside from structural stability, is that a clean crack is both easier to glue (hide glue likes fresh surfaces) and is less visible—no sweat, dust, or polish in there yet.

Glueing it today will be easier and look nicer than doing it later.
Author:  Ernie Kleinman [ Tue Feb 26, 2019 8:35 am ]
Post subject:  Re: When to repair a hairline crack in a top or back?
May I suggest as an alternative to hide, fish glue, for hairline cracks , clean up with water and enough lead time 1/2 hr to add necessary cleats or clamps. There is a demo on youtube by a cdn luthier twoodford ?? or something similiar he does a lot of very clever an innovative repair techniques
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